Access to cellular components such as nucleic acids is imperative to a variety of molecular biology methodologies. Such methodologies include nucleic acid sequencing, direct detection of particular nucleic acid sequences by nucleic acid hybridization and nucleic acid sequence amplification techniques.
The preparation and purification of high-purity double-stranded (ds) plasmid DNA, single-stranded (ss) phage DNA, chromosomal DNA, agarose gel-purified DNA fragments and RNA is of critical importance in molecular biology. Ideally, a method for purifying nucleic acids should be simple, rapid and require little, if any, additional sample manipulation. Nucleic acids rendered by such a method should be immediately amenable to transformation, restriction analysis, litigation or sequencing. A method with all of these features would be extremely attractive in the automation of nucleic acid sample preparation, a goal of research and diagnostic laboratories.
Typically, the preparation of plasmid DNA from crude alcohol precipitates is laborious, most often utilizing CsCl gradients, gel filtration, ion exchange chromatography, or RNase, proteinase K and repeated alcohol precipitation steps. These methods also require considerable downstream sample preparation to remove CsCl and other salts, ethidium bromide and alcohol. Similar arguments extend when using any of these methods for purifying DNA fragments. A further problem with these methods is that small, negatively-charged cellular components can co-purify with the DNA. Thus, the DNA can have an undesirable level of contamination.
Nucleic acids can also be purified using solid phases. Conventional solid phase extraction techniques have utilized surfaces which either (1) fail to attract and hold sufficient quantities of nucleic acid molecules because of surface design to permit easy recovery of the nucleic acid molecules during elution, or (2) excessively adhere nucleic acid molecules to the surface, thereby hindering recovery of the nucleic acid molecules during elution. Conventional metal surfaces which cause these problems when utilized in solid phase extraction include certain silica surfaces such as glass and Celite. Adequate binding of nucleic acids to these types of surfaces can be achieved only by utilizing high concentrations of chaotropes or alcohols which are generally toxic, caustic, and/or expensive. For example, it is known that DNA will bind to crushed glass powders and to glass fiber filters in the presence of chaotropes. The chaotropic ions typically are washed away with alcohol, and the DNAs are eluted with low-salt solutions or water. Importantly, RNA and protein do not bind. However, a serious drawback in the use of crushed glass powder is that its binding capacity is low. In addition, glass powders often suffer from inconsistent recovery, incompatibility with borate buffers and a tendency to nick large DNAs. Similarly, glass fiber filters provide a nonporous surface with low DNA binding capacity. Other silicas, such as silica gel and glass beads, are not suitable for DNA binding and recovery. Currently, the solid phase of choice for solid phase extraction of DNA is Celite such as found in Prep-A-GeneT.TM. by Bio-Rad Laboratories. As with the crushed glass powders, high concentrations of chaotropes are required for adequate binding of the DNA to the Celite.
However, the hydration of silica substances has, in some instances resulted in elimination of the need for such high concentrations of chaotropes to elute bound DNA from the silica substance as taught in references such as EP 0 512 767, EP 0 585 660, U.S. Pat. No. 5,674,997 and EP 0 832 897.
There are numerous protocols for purifying DNA. For example, U.S. Pat. No. 4,923,978 discloses a process for purifying DNA in which a solution of protein and DNA is passed over a hydroxylated support and the protein is bound and the DNA is eluted. U.S. Pat. No. 4,935,342 discloses purification of DNA by selective binding of DNA to anion exchangers and subsequent elution. U.S. Pat. No. 4,946,952 discloses DNA isolation by precipitation with water-soluble ketones. A DNA purification procedure using chaotropes and dialyzed DNA is disclosed in U.S. Pat. No. 4,900,677.
Diatoms have also been utilized for purification of nucleic acids as evidenced by U.S. Pat. No. 5,234,809 to Boom et al. and U.S. Pat. No. 5,075,430 to Little et al.
Yet a further technique utilized for purification of nucleic acids is binding to specifically adapted paramagnetic particles. Examples of such techniques may be found in references such as European Specification EP 0 446 260 B1 and U.S. Pat. No. 5,512,439 (Homes et al.) which describe monodisperse, superparamagnetic particles having a particle diameter standard deviation of less than 5%. Each particle carries a plurality of molecules of an oligonucleotide, with each oligonucleotide having a section serving as a probe for a target nucleic acid molecule of interest.
U.S. Pat. No. 4,672,040 (Josephson) and U.S. Pat. No. 4,695,393 (Whitehead et al.) describe magnetically responsive particles for use in systems to separate certain molecules. The particles have a metal oxide core surrounded by a stable silicone coating to which organic and/or biological molecules may be coupled.
U.S. Pat. No. 3,970,518 (Giaever) describes a method of sorting and separating a select cell population from a mixed cell population. The method utilizes small magnetic particles which are coated with an antibody to the select cell populations.
U.S. Pat. No. 4,141,687 (Forrest et al.) describes an automatic apparatus and method to assay fluid samples. The apparatus utilizes a particulate material with a reagent bound thereto. The particulate material is magnetic, and the reagent is a substance which takes part in a reaction in the reaction mixture.
U.S. Pat. No. 4,230,685 (Senyei et al.) describes a method for magnetic separation of cells. The method utilizes magnetically-responsive microspheres which are coated with staphylococcal Protein A to which is bound antibody.
U.S. Pat. No. 4,774,265 (Ugelstad et al.) describes a process for preparing magnetic polymer particles. The particles are compact or porous polymer particles treated with a solution of iron salts.
U.S. Pat. No. 5,232,782 (Charmot) describes magnetizable "core-shell" microspheres which have a core of a magnetizable filler and a shell of crosslinked organopolysiloxane.
U.S. Pat. No. 5,395,688 (Wang et al.) describes magnetically responsive fluorescent polymer particles which have a polymeric core coated evenly with a layer of polymer containing magnetically responsive metal oxide.
U.S. Pat. No. 5,491,068 and U.S. Pat. No. 5,695,946 (Benjamin et al.) describe an assay method for detecting the presence of bacteria using magnetic beads with specific antibodies immobilized thereon.
U.S. Pat. No. 5,536,644 (Ullman et al.) describes a particle separation method. The method utilizes magnetic particles with surface functional groups, and optionally, an additional surface coating.
European Patent Specification EP 0 444 120 B1 (Homes et al.) describes a method for detection of target RNA or DNA. The method utilizes magnetic particles carrying a single stranded 5'-attached DNA probe capable of binding the target RNA or DNA.
International Publication No. WO 96/18731 (Deggerdal et al.) describes a method for isolating nucleic acid from a sample using a particulate solid support and an anionic detergent.
U.S. Pat. No. 5,705,628 (Hawkins) describes a method for DNA purification and isolation using magnetic particles with functional group-coated surfaces.